Modern semiconductor integrated circuits usually involve multiple layers separated by dielectric (insulating) layers, such as of silicon dioxide or silica, often referred to simply as an oxide layer, although other materials are being considered for the dielectric. The layers are electrically interconnected by holes penetrating the intervening oxide layer which contact some underlying conductive feature. After the holes are etched, they are filled with a metal, such as aluminum, to electrically connect the bottom layer with the top layer. The generic structure is referred to as a plug. If the underlying layer is silicon or polysilicon, the plug is a contact. If the underlying layer is a metal, the plug is a via.
Plugs have presented an increasingly difficult problem as integrated circuits are formed with an increasing density of circuit elements because the feature sizes have continued to shrink. The thickness of the oxide layer seems to be constrained to the neighborhood of 1 .mu.m, while the diameter of the plug is being reduced from the neighborhood of 0.25 .mu.m or 0.35 .mu.m to 0.18 .mu.m and below. As a result, the aspect ratios of the plugs, that is, the ratio of their depth to their minimum lateral dimension, is being pushed to 5:1 and above.
Filling such a hole with a metal presents two major difficulties.
The first difficulty is filling such a hole without forming an included void, at least with a filling process that is fast enough to be economical and at a low enough temperature that doesn't degrade previously formed layers. Any included void decreases the conductivity through the plug and introduces a substantial reliability problem. Chemical vapor deposition (CVD) is well known to be capable of filling such narrow holes with a metal, but CVD is considered to be too slow for a complete filling process. Physical vapor deposition (PVD), alternatively called sputtering, is the preferred filling process because of its fast deposition rates. However, PVD does not inherently conformally coat a deep and narrow hole. A fundamental approach for applying PVD to deep holes is to sputter the metal on a hot substrate so that the deposited material naturally flows into the narrow and deep feature. This process is typically referred to as reflow. However, high-temperature reflow results in a high thermal budget, and in general the thermal budget needs to be minimized for complex integrated circuits. Further, even at high temperatures, the metal does not always easily flow into a very narrow aperture.
The second difficulty is that an aluminum contact is not really compatible with the underlying semiconductive silicon. At moderately high temperatures, such as those required for the reflow of aluminum into the narrow hole, aluminum tends to diffuse into the silicon and to severely degrade its semiconductive characteristics. Accordingly, a diffusion barrier needs to be placed between the aluminum and the underlying silicon.
These problems are well known and have been addressed by Xu et al. in U.S. patent application, Ser. No. 08/628,835, filed Apr. 5, 1996, incorporated herein by reference in its entirety, which is a continuation in part of U.S. patent application, Ser. No. 08/511,825, filed Aug. 7, 1995.
As shown in the cross-sectional view of FIG. 1, a contact hole 10 having an aspect ratio defined by its depth 12 and its width 14 is etched through a dielectric layer 16 to an underlying substrate 18, which in the more difficult situation includes a surface layer of silicon. In the hole filling process, the contact hole 10 is conformally coated with a titanium (Ti) layer 20, a titanium nitride (TiN) layer 22, and a graded (TiN.sub.x) layer 24, that is, the graded layer 24 begins at its bottom as TiN but its top portion is nearly pure Ti. These three layers form a tri-layer barrier 26, which provides both the conformality and the adhesion to the underlying layers, as well as sufficient wetting for the after deposited aluminum. The Ti layer 20, after siliciding at a sufficiently high annealing temperature, forms a good ohmic contact with the underlying silicon substrate 18. Thereafter, a metal layer 28 is sputter deposited into the hole 10 so as to fill it without voids. That is, the tri-layer barrier 26 sufficiently wets to the after filled aluminum that it readily flows into the hole 10 at a moderate temperature while the tri-layer barrier 26 nonetheless provide a sufficient diffusion barrier between the aluminum 28 and the underlying silicon 18.
According to Xu et al., the wetting quality of the three layers 20, 22, 24 is enhanced by depositing them in a high-density PVD reactor. On the other hand, they recommend that the aluminum layer 28 be sputter deposited in a conventional PVD chamber with a low plasma density. In particular, they recommend that the aluminum layer 28 be deposited as two layers in an improved two-step cold/warm version of a conventional sputtering process. In the first cold step, a seed layer 30 of aluminum is sputter deposited at a substrate temperature below 200.degree. C. so as to conformally coat the underlying barrier tri-layer 26 with a fairly uniform aluminum layer. In the second, warm step, a filling layer 32 of aluminum is sputter deposited at higher temperatures so as to reflow and fill the contact hole 10. An advantage of the tri-layer barrier 26 grown by ionized metal plating (IMP) is that the warm Al filling layer 32 can be filled at temperatures below 400.degree. C., even as low as 350.degree. C. according to the reported data. The warm layer 32 can be deposited at a fairly high rate so as to improve the system throughput. Because the two aluminum layers 30, 32 differ primarily in their different deposition temperatures, they are likely deposited within a single conventional PVD chamber capable only of developing a low-density plasma. Also, the two deposition can be performed continuously, with the temperature being ramped up during the deposition. As a result, the two Al layers 30, 32 have no clear boundary between them.
In the context of contact hole filling, a high-density plasma is defined in one sense as one substantially filling the entire volume it is in and having an average ion density of greater than 10.sup.11 cm.sup.-3 in the principal part of the plasma. The conventional plasma-enhanced PVD reactor produces a plasma of significantly lower ion density. Although high-density plasmas are available in a number of different types of reactors, they are preferably obtained in inductively coupled plasma reactor, such as the type shown in schematical cross section in FIG. 2. For reasons to be described shortly, this is referred to an ionized metal plasma or plating (IMP) reactor.
As shown in this figure, which is meant only to be schematical, a vacuum chamber 40 is defined principally by a chamber wall 42 and a target backing plate 44. A PVD target 46 is attached to the target backing plate 44 and has a composition comprising at least part of the material being sputter deposited. For the deposition of both titanium (Ti) and titanium nitride (TiN), the target 46 is made of titanium. A substrate 48 being sputter deposited with a layer of a PVD film is supported on a pedestal electrode 50 in opposition to the target 46. Processing gas is supplied to the chamber 40 from gas sources 52, 54 as metered by respective mass flow controllers 56, 58, and a vacuum pump system 60 maintains the chamber 40 at the desired low pressure.
An inductive coil 62 is wrapped around the space between the target 46 and the pedestal 50. Three independent power supplies are used in this type of inductively coupled sputtering chamber. A DC power supply 64 negatively biases the target 46 with respect to the pedestal 50. An RF power source 66 supplies electrical power in the megahertz range to the inductive coil 62. The DC voltage applied between the target 46 and the substrate 48 causes the processing gas supplied to the chamber to discharge and form a plasma. The RF coil power inductively coupled into the chamber 40 by the coil 62 increases the density of the plasma, that is, increases the density of ionized particles. Magnets 68 disposed behind the target 46 significantly increase the density of the plasma adjacent to the target 46 in order to increase the sputtering efficiency. Another RF power source 70 applies electrical power in the frequency range of 100 kHz to a few megahertz to the pedestal 50 in order to bias it with respect to the plasma.
Argon from the gas source 54 is the principal sputtering gas. It ionizes in the plasma, and its positively charged ions are attracted to the negatively biased target 46 with enough energy that the ions sputter particles from the target 46, that is, target atoms or multi-atom particles are dislodged from the target. The sputtered particles travel primarily along ballistic paths, and some of them strike the substrate 48 to deposit upon the substrate as a film of the target material. If the target 46 is titanium or a titanium alloy and assuming no further reactions, a titanium film is thus sputter deposited, or in the case of an aluminum target, an aluminum film is formed.
For the sputter deposition of TiN in a process called reactive sputtering, gaseous nitrogen is also supplied into the chamber 40 from the gas source 52 along with the argon. The nitrogen chemically reacts with the surface layer of titanium being deposited on the substrate to form titanium nitride.
As Xu et al. describe in the cited patent application, a high-density plasma, primarily caused by the high amount of coil power applied to the chamber 40, increases the fraction of the sputter species that become ionized as they traverse the plasma, hence the term ionized metal plating. The wafer bias power applied to the pedestal 50 causes the pedestal 50 to become DC biased with respect to the plasma, the voltage drop occurring in the plasma sheath adjacent to the substrate 48. Thus, the bias power provides a tool to control the energy and directionality of the sputter species striking the substrate 48.
Xu et al. disclose that the Ti/TiN/TiN.sub.x barrier tri-layer 26 should be deposited in an ionized metal plating (IMP) process in which the various power levels are set to produce a high-density plasma. They observe that an IMP barrier tri-layer 26 as shown in FIG. 1, when deposited in the contact hole 10, promotes the reflow of aluminum into the contact hole 10 when the aluminum is subsequently deposited in a conventional PVD reactor, that is, one not using inductively coupled RF power and not producing a high-density plasma. This superior reflow is believed to require two characteristics in a narrow aperture. The barrier layer needs to adhere well to the underlying SiO.sub.2 or Si so as to form a continuous, very thin film. The aluminum needs to wet well to the barrier layer so that it flows over the barrier layer at relatively low temperatures.
Although the TiN IMP barrier tri-layer offers significant advantages in promoting reflow of subsequently deposited conventional PVD aluminum, as processing requirements become even more demanding, further improvement of reflow into narrow apertures is desired.